If you describe the weather or the atmosphere in a room as oppressive, you mean that it is unpleasantly hot and damp. The oppressive afternoon heat had quite tired him out = stifling

weighing heavily on the senses or spirit; "the atmosphere was oppressive"; "oppressive sorrows"

If you describe a society, its laws, or customs as oppressive, you think they treat people cruelly and unfairly. The new laws will be just as oppressive as those they replace. refugees from the oppressive regime. = repressive

To oppress people means to treat them cruelly, or to prevent them from having the same opportunities, freedom, and benefits as others. These people often are oppressed by the governments of the countries they find themselves in We are not normal like everybody else. If we were they wouldn't be oppressing us